In order to make a shaped foil or film workpiece, for instance a multipocket tray for holding yogurt or other foodstuffs, it is standard to produce a planar thermoplastic foil, e.g. of polyvinyl chloride or polypropylene, and then grip it between two dies, a flat upper one and a lower one formed with upwardly open cavities as it moves downstream in steps. Suction applied to the cavities pulls down the foil to form in it a plurality of pockets as the foil moves downstream in steps sandwiched between the upper and lower dies. As the foil moves downstream it cools and cures so that it can be pulled out of the lower-die cavities as a semifinished workpiece. During the cooling/curing phase, the upper die can be pulled back and returned upstream. Once the upper die is back in its upstream position the lower die can be recirculated back to mate with it to restart the cycle with another section of the foil.
The problem with such systems, as described in U.S. Pat. No. 5,980,231 of Arends and German Utility Model 81 15 644, is that when working at high production rates there is insufficient time for the workpiece to cure to a dimensionally stable amount before it is stripped out of the lower die, because the step interval is all that is available for curing of the foil. Thus the foil can shrink or deform and become unusable.